Method of manufacturing thyristor with integrated power supply for an associated circuit

ABSTRACT

A thyristor having a p-n-p-n semiconductor construction has a shoulder of the n-base extending through the p-base to an exterior surface of the body of the thyristor and is connected to an associated circuit to supply the circuit with operating current. The shoulder has a first part at the boundary surface of the body and a second part with smaller cross-sectional dimensions, connected between the first part and the n-base. The cross-sectional dimensions of the conductive connection between the shoulder and the external circuit are less than the corresponding dimensions of the first part, such that the conductive coating is shielded from the space charge zone adjacent the second part.

BACKGROUND

The present invention relates to a thyristor used with an associatedcircuit supplied with operating current from the thyristor, and moreparticularly to such a thyristor which has a shoulder of its n-baseprojecting through its p-base to the cathode side with a connection fromthe shoulder to an external circuit.

A thyristor of this general type is disclosed in German patentpublication No. A-32 26 613. The associated circuit comprises alight-sensitive switch element connected between the conductive coatingand the cathode of the thyristor. The switch element, such as forexample a phototransistor, exhibits a relatively low blocking capabilityand a high photosensitivity, so that the overall system including thethyristor and the switch element comprises both a great triggersensitivity as well as a great blocking capability. A disadvantage ofthis arrangement, however, is that the conductive coating, with a highblocking voltage, is close to the region of the space charge zone at thep-n junction between the two base layers, which space charge zonelargely fills out the shoulder of the n-base. This leads to leakagecurrents which can bring about an unintentional ignition of thethyristor under some conditions, or which can make a desired ignitionmore difficult under other conditions.

BRIEF DESCRIPTION OF THE INVENTION

A principal object of the present invention is to provide a thyristor ofthe type described wherein such leakage currents are largely avoided anda reliable and controllable power supply is provided for the associatedcircuit. This is achieved in the present invention by arranging theshoulder of the n-base to have a first part located at the boundarysurface which is larger in both dimensions parallel to the boundarysurface than the conductive coating by which the shoulder is connectedto an external circuit, and a second part connecting the first partthrough the p-base to the main part of the n-base, with thecross-sectional dimensions of the second part being less than thecorresponding dimensions of the first part, to isolate the conductivecoating from the space charge zone between the shoulder and the p-base.

The present invention obtains the advantage that the associated circuitis supplied with the required electrical power in a controllable way, sothat both the maximum operating voltage and maximum operating currentsupplied thereto are prescribed by an appropriate dimensioning of thesecond part of the shoulder.

These and other advantages of the present invention will become manifestby inspection of the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a first exemplary embodiment of theinvention with an associated circuit for light triggering of thethyristor;

FIG. 2 is a voltage diagram serving to explain operation of theapparatus of FIG. 1;

FIG. 3 is a cross-sectional diagram of a second exemplary embodiment ofthe invention using a logic circuit for processing the signals of atemperature sensor as the associated circuit unit;

FIG. 4 is a cross-section of a further embodiment of the presentinvention illustrating a method of making the thyristors of FIGS. 1 and3; and

FIG. 5 is a cross-section of a further embodiment of the inventionillustrating another method of making the thyristors of FIGS. 1 and 3.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows a thyristor comprising a semiconductor body of dopedsemiconductor material, for exampIe silicon, which comprises foursuccessive layers of alternating conductivity types. These are then-emitter 1, the p-base 2, the n-base 3 and the p-emitter 4. Then-emitter 1 is provided with an electrode 5 at the cathode side and thep-emitter 4 is provided with an electrode 6 at the anode side. Theelectrodes are composed of an electrically conductive material, forexample aluminum. The electrode 5 is connected to ground via a terminalK; the electrode 6 is connected to the upper terminal 9 of a voltagesource 10 via a terminal A and a load resistor R, the lower terminal 11of said voltage source 10 being connected to at ground.

The n-base has a n-conductive shoulder 12 which penetrates the p-base 2,extends up to the boundary surface 1a of the semiconductor body, and iscontacted there by a conductive coating 13. The shoulder 12 is composedof a first part 14 and of a second part 15 which is arranged between thepart 14 and the n-base 3. The cross-section of the part 15 can, forexample, have a quadratic or rectangular shape, and the visibledimension B1 is dimensioned greater than the corresponding dimension ofthe coating 13. The width of the part 14 measured perpendicular to theplane of the drawing, is likewise dimensioned greater than the dimensionof the coating 13 in this direction. Further, the cross-sectionaldimensions of the second part 15 likewise comprise a quadratic orrectangular cross section, i.e. the depth 2a measured in the plane ofthe drawing, and the width Z measured perpendicular thereto, aredimensioned smaller than the corresponding dimensions of the part 14.The length of the part 15 is referenced L in the drawings.

The n-emitter 1 is provided with emitter-base short circuit paths whichare composed of shoulders 16 and 17 of the p-base 2 which penetrate then-emitter 1 and are contacted by the electrode 5 at the boundary surface1a. The coating 13 has a terminal S which is connected to the firstterminal of a circuit unit 18 associated with the thyristor. Thiscontains a light-sensitive switch element which is preferably followedby an amplifier stage. The simplest case, the unit 18, as indicated inFIG. 1, is composed of a phototransistor 20 whose terminals 21 and 22are respectively connected to S and G, so that the terminals S and G areconnected to one another via the switching path of the phototransistor20.

The second part 15 of the shoulder 12 can be interpreted as the channelof an intrinsic layer field effect transistor (JFET) comprising achannel depth 2a, a channel length L and a channel width Z whose sourceterminal is formed by S and whose drain is formed by the n-base 3. Thep-base surrounding the channel 15 represents a gate region that iscontacted by 19 and is controllable via the terminal G. Via the shortcircuit paths 16 and 17, G always lies at the ground potential of theelectrode 5 at the cathode side.

When it is assumed that the light-sensitive switch element of thecircuit unit 18 is not illuminated, then the connection between S and Gis extremely high in resistance. When the anode voltage V_(AK) is thenallowed to rise from zero in the direction toward positive values, thenthe potential at the terminal S and at a point D, which roughlyidentifies the junction between the channel 15 and the n-base 3,corresponds initially to the voltage V_(AK). When V_(AK) reaches a valueV_(P) at which the full cross-se'ction of the channel 15 is filled outby the space charge zone, which builds up at the p-n junction 23 betweenthe shoulder 12 and the p-base 2, then the circuit points S and D alsolie at V_(P). A further rise V_(AK), for example to 5000 volts andabove, then no longer influences the voltage at S and D. When thevoItage V_(SK) between the terminals S and K is entered over the anodevoltage V_(AK), then this condition can be illustrated in accord withFIG. 2, where the abcissa is V_(AK) and the ordinate is V_(SK), reachingthe limit value V_(P)

As may be derived from the book by S. M. Sze, "Physics of SemiconductorDevices", Wiley & Sons, New York, 1969, cf. pp. 340-351, the value ofV_(P) can be determined by a corresponding selection of the dopingconcentration N_(D) of the shoulder 12, and of half the channel depth a,in accord with the relationship: ##EQU1## where Q represents theelementary charge, and ε represents the dielectric constant of thesemiconductor material. With N_(D) =2.10¹³ cm⁻³ and a=5.10⁻³ cm, a valueof about 40 volts is determined for V_(P).

When, given an anode voltage V_(AK) exceeding V_(P), the terminals S andG are connected to one another in low resistance fashion, then the drainvoltage V_(D) =V_(P) for the JFET, and its gate voltage V_(G) =0(respectively referred to S). The maximum drain current I_(Sat) iscalculated by the relationship: ##EQU2## I_(P) is defined by therelationship ##EQU3## where V_(bi) denotes the impressed voltage at thep-n junction between the parts 2 and 15, and μ denotes the chargecarrier mobility in the channel 15. I_(P) can be set independently bymeans of the channel Z of the part 15. With the above values and Z=0.3cm as well as L=5.10⁻¹³ cm, for example, a value of about 25 mA resultsfor I_(P).

In the structure of FIG. 1, the shoulder 12 has the significance of anintegrated power supply part which can supply the circuit unit 18 with amaximum operating current I_(DSat) when V_(AK) rises above V_(P). Whenthe light-sensitive switch element in 18 is illuminated, then, given thestructure design set forth above, a drain current flows, up to 25 mA,dependent on the light intensity and the gain of the circuit unit 18.This is supplied to the terminal G of the trigger electrode 19 astrigger current. The light energy which is supplied to the circuit unit18 for triggering the ignition is significantly lower than that whichwould have to be supplied to the thyristor for direct light ignition.

The cross-sectional dimensions of the first part 14 of the shoulder 12are selected to be greater than the dimensions of the coating 13,because this guarantees that the coating 13 does not come into contactwith the space charge zone forming at the p-n junction 23. Disturbingleakage currents are avoided in this way.

FIG. 3 shows another exemplary embodiment of the invention whichcompletely corresponds to the semiconductor structure of FIG. 1 butwhich differs by the use of a different wiring of S, G and K. A circuitunit 24 associated with the thyristor is provided which is supplied withoperating current from the load circuit connected at A and K, beingsupplied therewith via the integrated power supply part 12. A logiccircuit 24 is connected via a line 26 to a temperature sensor 25 locatedat the boundary surface 1a. The temperature sensor 25 supplies a signalwhen the semiconductor body of the thyristor reaches a maximum allowabletemperature, or has exceeded it. This signal is processed into a switchsignal by the circuit 24, and this switch signal opens an electronicswitch 27 so that an ignition current pulse 28 which is supplied by anignition current circuit (not shown) cannot reach the terminal G.Ignition of the thyristor is thus suppressed until its temperature hasagain dropped below the maximum allowable value. The logic circuit 24can also be fashioned as an alarm logic which, when the criticaltemperature is reached, communicates an alarm related thereto to amonitoring location, as indicated by the arrow 29. The operating currentfor the logic circuit 24 is supplied via the terminals 24a connected toS and K.

The manufacture of the thyristor structure of FIGS. 1 and 3 is carriedout in a traditional way by means of a series of diffusion steps whichare executed upon application of diffusion masks. FIG. 4 shows amodification of this thyristor structure which can be manufactured in asimpler way. The basis is thereby formed by a wafer of n-dopedsemiconductor material, for example silicon. Before the execution of adeep p-diffusion forming the p-base 2 and the p-emitter 4, the boundarysurface 1a in the region of the coating 13, is covered by a diffusionmask and a deep notch 29a proceeding from the boundary surface 4a isapplied at the other side of the wafer. After the p-diffusion, thesemiconductor wafer comprises p-regions at the edge which are limited bythe lines 30 from the inner wafer part. An indiffusion of the n-emitter1 into the p-base 2 subsequently is performed. When the part of thewafer shown with broken lines is removed, up to the line 32, by bevelingthe p-n junction at the anode side, then a structure is produced whichfundamentally corresponds to the thyristor structure of FIG. 1. Indetail, the part 14' corresponds to the first part 14 of the shoulder 12of FIG. 1, whereas the channel 15 of FIG. 1 is replaced by the part 15',whose length and depth are again referenced L and 2a. After theapplication of the coatings 13, 19, 5 and 6, the terminals S, G, K and Acan be wired in accord with FIG. 1 or FIG. 3.

Given a cylindrical-symmetrically formation of the thyristor having anaxis of symmetry 31, the notch 29a can also have a cylindrical shape.When, by contrast, the notch proceeds on a straight line from wafer edgeto wafer edge, then the part of the wafer edge lying to the rightthereof must be beveled to such degree that the lateral limiting surfaceproceeds through the n-doped wafer region which lies outside the part14' in radial direction of the wafer.

FIG. 5 shows a thyristor which is also manufactured in the way set forthabove but with the departure that, instead of a notch 29a, a part of thesemiconductor wafer bounded by the lines 33 and 34 is now removed. Uponexecution of the p-diffusion, p-regions at the edge side are produced,which are limited from the interior of the wafer by the lines 30'. Whenthat part of the semiconductor wafer indicated with broken lines is thenremoved to the line 35, by beveling the p-n junction at the anode side,then a structure comparable to FIG. 1 again arises, whereby the parts14" and 15"--the latter comprising the length L and the depth2a--correspond to the parts 14 and 15. The terminals S, G, K and A canagain be wired in accord with FIG. 1 or FIG. 3. Given acylindrical-symmetrical fashioning of the thyristor, an annular part oft he wafer whose cross section is limited by 33, 34 can also be removed.The line 35 then denotes a part of a conical surface. When, however, awafer part which is limited by two planar surface parts lyingperpendicular to the plane of the drawing is removed, these surfaceparts corresponding to the horizontal line 33 and the vertical line 34,then that part of the wafer edge lying to the right of the vertical line34 must again be beveled such that the lateral limiting surface proceedsthrough the n-doped wafer region which lies outside of the part 14" inthe radial direction of the wafer.

The scope of the invention embraces further embodiments wherein aplurality of shoulders 12 are arranged next to one another and areprovided with allocated coatings 13. As a result thereof a largermaximum operating current maY be supplied to the circuit unit 18 or 24.A respective n-doped auxiliary emitter (amplifying gate) fashioned in atraditional way can be provided between the coating 19 and the n-emitter1, this auxiliary emitter being provided with a conductive coating onthe surface 1a, which coating projects beyond the p-n junction in thedirection toward the n-emitter 1.

The circuit units 18 and 24 set forth can be replaced by any otherarbitrary circuit units associated with the thyristor, which circuitsare then supplied with operating electrical power acquired via theshoulder 12 from the load circuit of the thyristor. These include, forexample, logic circuits which process the signals of sensors respondingto excess anode voltages, excess rates of voltage rise or excess loadcurrents.

What is claimed is:
 1. A method for the manufacture of a thyristor having a p-emitter, an n-base, p-base, and an n-emitter being interconnected between anode and cathode electrodes respectively, and having a shoulder of said n-base extending through said p-base to an external electrical contact, comprising the steps of performing a deep diffusion of said p-base and said p-emitter in a wafer of n-doped semiconductor material using a doping mask covering the region above said shoulder at the cathode side of said thyristor making a deep notch in the surface of said thyristor at the anode side at a location opposite said shoulder, inserting said n-emitter into said p-base, and beveling the p-n junction at the anode side of said body such that the p-doped semiconductor region adjacent the lateral faces of said notch does not constitute a conductive connection to said p-emitter.
 2. A method for the manufacture of a thyristor having a p-emitter, and n-base, p-base, and an n-emitter being interconnected between anode and cathode electrodes respectively, and having a shoulder of said n-base extending through said p-base to an external electrical contact, comprising the steps of performing a deep p-diffusion in a body of n-doped semiconductor material using a doping mask covering the region above the shoulder of said n-base at the boundary surface on the cathode side of said body, removing a part of said semiconductive body from a location lying opposite said shoulder, inserting said n-emitter into said p-base, and beveling the p-n junction at the anode side of said body such that the p-doped semiconductor region adjacent the recess caused by removal of said removed part does not constitute a conductive connection to said p-emitter. 